I've never really thought of conflicts in their basic forms like that.

In general I find internal conflicts more difficult, although more interesting. It can be tempting to simply write out the character's secret thoughts to show what's going on inside them but I like to keep that to an absolute minimum and show it through their actions instead.

In my WIP, the most difficult one could probably be summarised as man vs. his desires. In a romance we all know how it will end up (with him giving in to his desire for the love interest) so I have to make the struggle realistic AND not tedious or frustrating for the reader. Not easy.